communication eng

1. Chapter 4
FREQUENCY MODULATION

2. INTRODUCTION
 3 properties of an analog signal can be modulated by
information signal:
o Amplitude - - -> produce AM
o Frequency ---> produce FM
o Phase ---> produce PM
 FM & PM are forms of angle modulation and often
referred as frequency modulation.

3. FM VS AM
 FM is considered to be superior to AM.
 Transmission efficiency:
 AM use linear amplifier to produced the final RF signal.
 FM has constant carrier amplitude so it is not necessary to use
linear amplifier.
 Fidelity (capture effect):
 The stronger signal will be capture and eliminate the weaker.
 In AM, the weaker signal can be heard in the background.
 Noise immunity (noise reduction):
 Constant carrier amplitude.
 FM receiver have limiter circuit

4. Disadvantages of FM
 Use too much spectrum space.
 Requiring a wider bandwidth
 Reduce modulation index to minimize BW but in FM
although we reduced the modulation index, BW is still larger.
 typically used at high frequencies (VHF,UHF & microwave
frequencies
 More complex circuitry

5. ANGLE MODULATION
 Amplitude of the modulated carrier is held constant and either the
phase or the time derivative of the phase of the carrier is varied linearly
with the message signal m(t).
 General angle-modulated signal is given by
m( t ) = Vc cos[ωct + θ ( t ) ]
 In angle modulation, θ(t) is prescribed as being a function of the
modulating signal
 If vm(t) is the modulating signal, angle modulation is expressed as
θ (t ) = F [ vm (t ) ]
vm (t ) = Vm sin(ωmt )
where
ωm = 2π f m

6. FM OR PM ?
FM PM
Instantaneous frequency of the carrier is Phase angle of the carrier is varied
varied from its reference value by from its reference value by an
an amount proportional to the amount proportional to the
modulating signal amplitude modulating signal amplitude
Freq. carrier - - - > directly varied Phase carrier - - - > directly varied
Phase carrier - - -> indirectly varied Freq. carrier - - -> indirectly varied
 Both must occur whenever either form of angle modulation is
performed.

7. 2∆f
∆f ∆f
fc-∆f fc fc+∆f f
vm(t) = Vm cos 2πfmt
-Vm 0 +Vm
Figure 4.1 : Frequency deviation

8. MATHEMATICAL ANALYSIS
 Instantaneous frequency deviation
 Instantaneous change in the frequency of the carrier and is defined
as the first time derivative of the instantaneous phase deviation
instantaneous frequency deviation = θ '(t ) rad/s
θ '(t ) rad/s cycle
or = = = Hz
2π rad/cycle s
 Instantaneous frequency
 the precise frequency of the carrier at any given instant of time and
is defined as the first time derivative of the instantaneous phase
d
instantaneous frequency = ωi (t ) = [ ωct + θ (t )]
dt
= ωc + θ '(t ) rad/s

9.  Substituting 2πfc for ωc gives
instantaneous frequency = fi (t )
 rad   cycles 
and ωi (t ) =  2π   fc  + θ '(t ) = 2π f c + θ '(t ) rad/s
 cycle   s 
 Frequency modulation is angle modulation in which the
instantaneous frequency deviation, θ’(t), is proportional to
the amplitude of the modulating signal, and the
instantaneous phase deviation is proportional to the integral
of the modulating signal voltage.

10. DEVIATION SENSITIVITY
 For modulating signal vm(t), the frequency modulation are
frequency modulation = θ’(t) = kfvm(t) rad/s
where kf are constant and are the deviation sensitivities of the
frequency modulator.
 Deviation sensitivities are the output-versus-input transfer
function for the modulators, which gave the relationship
between what output parameter changes in respect to
specified changes in the input signal.
 frequency modulator,
rad/s  ∆ω 
kf =  
V  ∆V 

11. FREQUENCY MODULATION
(FM)
 Variation of dθ /dt produces Frequency
Modulation
 Frequency modulation implies that dθ/dt is
proportional to the modulating signal.
 This yields v (t ) = V sin [ ω t + θ (t )]
FM c c
= Vc sin ωc t + ∫ θ '(t )dt 
 
= Vc sin ωc t + ∫ k f vm (t )dt 
 
= Vc sin ωc t + k f Vm ∫ sin ωm (t ) dt 
 
 k f Vm 
= Vc sin ωc t − cos ωm (t ) 
 ωm 

12. Example 4.1
Derive the FM signal using both cosine wave
signal.
v(t ) = Vc cos ( ωc t + θ (t ) )
vm (t ) = Vm cos ( ωmt )
for FM
for PM
(( ))
vFM ((t))= Vc c cosωωc t ∫ kk p vmt()tdt
PM
t = Vcos c t + + f vm ( )
= V cos ( ωω + ∫ kk V cos(ωω )dt )
= V cos ( t t + V cos( t t )
cc c c f p mm m m
= V cos ( ω t + k V ∫ cos(ω t )dt )
c c f m m
 k f Vm 
= Vc cos  ωc t + sin(ωmt ) 
 ωm 

13. FM WAVEFORM
Figure 4.2: Phase and Frequency modulation ; (a) carrier signal (b) modulating
signal (c) frequency modulated wave (d) phase modulated wave

14.  Carrier amplitude remains constant
 Carrier frequency is changed by the modulating signal.
 amplitude of the information signal varies, the carrier frequency shift
proportionately.
 modulating signal amplitude increases, the carrier frequency increases.
 modulating signal amplitude varies, the carrier frequency varies below and
above it normal center or resting, frequency with no modulation.
 The amount of the change in carrier frequency produced by the
modulating signal known as frequency deviation fd.
 Maximum frequency deviation occurs at the maximum amplitude
of the modulating signal.
 The frequency of the modulating signal determines the frequency
deviation rate

15. MODULATION INDEX
 Directly proportional to the amplitude of the modulating signal
and inversely proportional to the frequency of the modulating
signal
 Ratio of the frequency deviation and the modulating frequency
 FM equation : vFM (t ) = Vc sin [ ωct − β cos ωm (t )]
 β as modulation index : k f Vm ∆f c
β= =
ωm fm
 Example:
 Determine the modulation index for FM signal with modulating frequency
is 10KHz deviated by ±10kHz.
 Answer : (20KHz/10KHz) = 2 .0 (unitless)
 The total frequency change, 10kHz x 2 is called the carrier swing

16. Example:
 a simple transmitter with an assigned rest frequency of 100MHz
deviated by a ±25kHz, the carrier changes frequency with modulation
between the limits of 99.975MHz and 100.025MHz
 The total frequency change, 25kHz x 2 is called the carrier swing
 Table 1 display the transmission band that use FM and the legal
frequency deviation limit for each category
 Deviation limits are based on the quality of the intended
transmissions, wider deviation results in higher fidelity
 The frequency deviation is a useful parameter for determining the
bandwidth of the FM-signals

17.  Table 1 display the transmission band that use FM and the legal
frequency deviation limit for each category
Specifications for transmission of FM signal

18. PERCENT MODULATION
 Simply the ratio of the frequency deviation actually
produced to the maximum frequency deviation allowed by
law stated in percent form
∆f actual
% modulation =
∆f max
 For example if a given modulating signal produces ±50kHz
frequency deviation, and the law stated that maximum
frequency deviation allowed is ±75kHz, then
50kHz
% modulation = × 100 = 67%
75kHz

19. Example 4.2
A 1 MHz carrier freq with a measured sensitivity of 3
kHz/V is modulated with a 2 V, 4 kHz sinusoid.
Determine
1. the max freq deviation of the carrier
2. the modulation index
3. the modulation index if the modulation voltage is
doubled
4. the modulation index for vm(t)=2cos[2π(8kHz)t)]V
5. express the FM signal mathematically for a cosine
carrier & the cosine-modulating signal of part 4. Carrier
amplitude is 10V

21. FM RADIO FREQUENCY
 Commercial radio FM band, 88MHz – 108MHz
 Each station allotted to a frequency deviation of
±75kHz (150 carrier swing) and 25kHz of guard
band added above and below the carrier
frequency swing
 Total bandwidth is 200kHz
 Therefore, maximum of 100 stations can be
made available

22. FREQUENCY
ANALYSIS OF FM
WAVES

23. BESSEL TABLE
,β
Tabulated value for Bessel Function for the first kind of the nth order

24.  The first column gives the modulation , while the first row gives the
Bessel function.
 The remaining columns indicate the amplitudes of the carrier and the
various pairs of sidebands.
 Sidebands with relative magnitude of less than 0.001 have been
eliminated.
 Some of the carrier and sideband amplitudes have negative signs. This
means that the signal represented by that amplitude is simply shifted in
phase 180° (phase inversion).
 The spectrum of a FM signal varies considerably in bandwidth
depending upon the value of the modulation index. The higher the
modulation index, the wider the bandwidth of the FM signal.
 With the increase in the modulation index, the carrier amplitude
decreases while the amplitude of the various sidebands increases. With
some values of modulation index, the carrier can disappear completely.

25. Bessel Function, Jn(m) vs m

26. PROPERTIES OF BESSEL
FUNCTION
 Property - 1: Property - 3:
∞
For n even,
∑ J n (β ) = 1
2
we have Jn(β) = J-n(β) n =−∞
For n odd,
we have Jn(β) = (-1) J-n(β)
Thus,
Jn(β) = (-1)n J-n (β)
 Property - 2:
For small values of the modulation index β, we have
J0(β) ≅ 1
J1(β) ≅ β/2
J3(β) ≅ 0 for n > 2

27. AMPLITUDE SPECTRUM
Amplitude spectrum of different value of β

28. FM BANDWIDTH
 The total BW of an FM signal can be determined by knowing the
modulation index and Bessel function.
BW = 2 f m N
N = number of significant sidebands
fm = modulating signal frequency (Hz)
 Another way to determine the BW is use Carson’s rule
 This rule recognizes only the power in the most significant
sidebands with amplitude greater than 2% of the carrier.

29. Example 4.3
Calculate the bandwidth occupied by a FM signal with a
modulation index of 2 and a highest modulating frequency of
2.5 kHz. Determine bandwidth with table of Bessel functions.
Referring to the table, this produces 4 significant pairs of
sidebands.
BW = 2 × 4 × 2.5
= 20kHz

30. CARSON’S RULE
BW = 2[ f d (max) + f m (max) ]
fd (max) = max. frequency deviation
fm (max) = max. modulating frequency
 Carson’s rule always give a lower BW calculated with the
formula BW = 2fmN.
 Consider only the power in the most significant sidebands
whose amplitudes are greater than 1% of the carrier.
 Rule for the transmission bandwidth of an FM signal
generated by a single of frequency fm as follows:
BT = BW ≅ 2 ∆f + 2 f m = 2 ∆f (1 + 1 )
β
or = 2 fm ( 1 + β )

31. Example 4.4
For an FM modulator with a modulation index β =
1, a modulating signal
vm(t) = Vmsin(2π1000t) and unmodulated carrier
vc(t) = 10sin(2π500kt), determine
d) Number of sets of significant sideband
e) Their amplitude
f) Then draw the frequency spectrum showing their
relative amplitudes

32. Example 4.5
For an FM modulator with a peak freq deviation Δf
= 10kHz, a modulating signal freq fm= 10kHz, Vc
=10V and 500kHz carrier, determine
b) Actual minimum bandwidth from the Bessel
function table
c) Approximate minimum bandwidth using Carson’s
rule
d) Plot the output freq spectrum for the Bessel
approximation

33. DEVIATION RATIO (DR)
 Minimum bandwidth is greatest when maximum freq
deviation is obtained with the maximum modulating
signal frequency
 Worst case modulation index and is equal to the
maximum peak frequency deviation divided by the
maximum modulating signal frequency
 Worst case modulation index produces the widest
output frequency spectrum
 Mathematically,
max peak freq deviation ∆f max
DR = =
max mod signal freq f m (max)

34. Example 4.6
• Determine the deviation ratio and bandwidth for
the worst case (widest bandwidth) modulation
index for an FM broadcast band transmitter with a
maximum frequency deviation of 75kHz and a
maximum modulating signal frequency of 15kHz
• Determine the deviation ratio and maximum
bandwidth for an equal modulation index with only
half the peak frequency deviation and modulating
signal frequency

35. POWER IN ANGLE-
MODULATED SIGNAL
 The power in an angle-modulated signal is easily computed
P = VC2/2R W
 Thus the power contained in the FM signal is independent
of the message signal. This is an important difference
between FM and AM.
 The time-average power of an FM signal may also be
obtained from
vFM (t ) = Vc cos(2π f c t + θ (t ))

36. Example 4.7
An FM signal is given as vFM(t)=12cos[(6π106t)
+ 5sin(2π x 1250t)] V. Determine
a. freq of the carrier signal
b. freq of the modulating signal
c. modulation index
d. freq deviation
e. power dissipated in 10 ohm resistor.

37. Example 4.8
Determine the unmodulated carrier power for the
FM modulator given that β =1, Vc=10 V, R = 50
Ω. Then, determine the total power in the angle-
modulated wave.
Solution:
 not exactly equal because values in Bessel
table have been rounded off.

38. Example 4.9
An FM signal expressed as v FM (t ) = 1000 cos(2π 10 7 t + 0.5 sin 2π 10 4 t )
is measured in a 50 ohm antenna. Determine the following :-
a. total power
b. modulation index
c. peak freq deviation
d. modulation sensitivity if 200 mV is required to achieve part c
e. amplitude spectrum
f. bandwidth (99%) and approximate bandwidth by Carson’s rule
g. power in the smallest sideband of the 99% BW
h. total information power

39. Example 4.10
An FM signal with 5W carrier power is
fluctuating at the rate of 10000 times per second
from 99.96 MHz to 100.04 MHz. Find
a. carrier freq
b. carrier swing
c. freq deviation
d. modulation index
e. power spectrum

40. Example 4.11
In an FM transmitter, the freq is changing between 100
MHz to 99.98 MHz, 400 times per seconds. The amplitude
of the FM signal is 5 V, determine :-
1. carrier and modulating freq
2. carrier freq swing
3. amplitude spectrum
4. bandwidth by using Bessel Table and Carson’s rule
5. average power at the transmitter if the modulator carrier
power is 5 W.

41. FM SIGNAL GENERATION
 They are two basic methods of
generating frequency-Modulated
signals:
 Direct Method
 Indirect Method

42. DIRECT FM
 In a direct FM system the instantaneous frequency is
directly varied with the information signal. To vary the
frequency of the carrier is to use an Oscillator whose
resonant frequency is determined by components that can
be varied. The oscillator frequency is thus changed by the
modulating signal amplitude.
f i = f c + k f vm (t )
• For example, an electronic Oscillator has an output
frequency that depends on energy-storage devices. There
are a wide variety of oscillators whose frequencies depend
on a particular capacitor value. By varying the capacitor
value, the frequency of oscillation varies. If the capacitor
variations are controlled by vm(t), the result is an FM

43. INDIRECT FM
 Angle modulation includes frequency modulation FM and
phase modulation PM.
 FM and PM are interrelated; one cannot change without the
other changing. The information signal frequency also
deviates the carrier frequency in PM.
 Phase modulation produces frequency modulation. Since
the amount of phase shift is varying, the effect is that, as if
the frequency is changed.
 Since FM is produced by PM , the later is referred to as
indirect FM.
 The information signal is first integrated and then used to
phase modulate a crystal-controlled oscillator, which
provides frequency stability.

44. NOISE AND PHASE SHIFT
 The noise amplitude added to an FM signal
introduces a small frequency variation or phase
shift, which changes or distorts the signal.
 Noise to signal ratio N/S
N Frequency deviation produced by noise
=
S Maximum allowed deviation
 Signal to noise ration S/N
S 1
=
N N
S

45. INTERFERENCE
 A major benefit of FM is that interfering signals on the
same frequency will be effectively rejected.
 If the signal of one is more than twice the amplitude of the
other, the stronger signal will "capture" the channel and will
totally eliminate the weaker, interfering signal.
 This is known as the capture effect in FM.
 In FM, the capture effect allows the stronger signal to
dominate while the weaker signal is eliminated.
 However, when the strengths of the two FM signals begin
to be nearly the same, the capture effect may cause the
signals to alternate in their domination of the frequency.

46.  Despite the fact that FM has superior noise rejection
qualities, noise still interferes with an FM signal. This is
particularly true for the high-frequency components in the
modulating signal.
 Since noise is primarily sharp spikes of energy, it contains a
considerable number of harmonics and other high-
frequency components.
 These high frequencies can at times be larger in amplitude
than the high-frequency content of the modulating signal.
 This causes a form of frequency distortion that can make
the signal unintelligible.
 To overcome this problem Most FM system use a
technique known as Pre-emphasis and De-emphasis.